1516 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 48, NO. 5, SEPTEMBER/OCTOBER 2012 Transition Control Strategy Between Standalone and Grid-Connected Operations of Voltage-Source Inverters Md. Nayeem Arafat, Student Member, IEEE, Sreeshailam Palle, Yilmaz Sozer, Member, IEEE, and Iqbal Husain, Fellow, IEEE Abstract—This paper proposes a smooth transition control strategy for voltage-source inverters between standalone (SA) and grid-connected (GC) modes of operation. In the GC mode, the amount of power exchanged with the utility grid is controlled by regulating the phase currents. In the SA mode, the load voltage is regulated by the inverter with its phase dictated by the inverter control. The transition between SA and GC operations that will ensure continuous power delivery to the load requires continuation in the phase of the system voltage. The proportional–integral, trapezoidal, sinusoidal, and staircase frequency variation tech- niques have been analyzed to find the best approach for mini- mizing the total harmonic distortion (THD). A smooth frequency variation technique has been developed, which provides lower THD on the voltage waveforms compared with the other tech- niques. The new algorithm has been implemented on a 5-kW single-phase utility interactive inverter having the SA operation capability. The simulation and experimental results show that the proposed technique provides seamless transition between the inverter modes of operation with minimal distortion at the inverter output voltage. Index Terms—AC–DC power converters, distributed power generation, uninterruptible power systems. I. I NTRODUCTION I N RECENT years, the demand for energy from renewable sources has increased to address the energy crisis and environmental pollution problems. This has resulted in the proliferation of renewable-based distributed energy sources for power generation into the grid; the penetration rate is expected to increase in the coming years. Renewable energy sources such as wind turbines, photovoltaic power systems, and regenerative fuel cells typically provide dc output power. Distributed gen- eration systems are connected to the utility grid (UG) through Manuscript received December 9, 2011; revised March 14, 2012; accepted March 18, 2012. Date of publication August 1, 2012; date of current version September 14, 2012. Paper 2011-EMC-543.R1, presented at the 2011 IEEE Energy Conversion Congress and Exposition, Phoenix, AZ, September 17–22, and approved for publication in the IEEE TRANSACTIONS ON I NDUSTRY APPLICATIONS by the Electric Machines Committee of the IEEE Industry Applications Society. Md. N. Arafat, S. Palle, and Y. Sozer are with the Electric and Computer Engineering Department, The University of Akron, Akron, OH 44325 USA (e-mail: mna9@zips.uakron.edu; sp62@zips.uakron.edu; ys@uakron.edu). I. Husain is with the Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, NC 27695 USA (e-mail: ihusain2@ ncsu.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIA.2012.2210013 Fig. 1. Single-phase utility-interactive inverter system. power electronic inverters. The utility interactive inverters play an important role in the power distribution system [1]–[6]. The grid-connected (GC) inverter should be able to operate in GC and standalone (SA) modes in order to provide power to the emergency load during outages. An unexpected power disrup- tion could cause injuries, fatalities, serious business disruption, or data loss. Recently, lots of the sensitive devices such as life- support equipment, instrumentation plants, satellites, hospital equipment, industrial controllers, microwave broadcasts, and television engine rooms [7]–[9] require clean input voltage with low total harmonic distortion (THD). For example, there is a specific power supply requirement from International Broad- cast Center to maintain continuous and stable power supply with low THD, which will allow uninterrupted sportscasts all around the world [10], [16]. The energy system used in this paper is the utility interactive inverter. In the case of utility outage, the system disengages itself from the grid and provides sustainable and high-quality power to the critical loads. The utility interactive inverter with the SA operation capabil- ity analyzed in this paper is shown in Fig. 1. The SA and GC modes of inverter operation require different control methods. Moreover, the transition between the two modes should be seamless to minimize any sudden voltage change across the emergency load or any sudden current change provided into the grid. The transitions between the two modes should be fast and precise to minimize the interruption in the power supply. 0093-9994/$31.00 © 2012 IEEE